Defects and some physical properties of nominally pure and zinc-doped lithium niobate crystals

The IR absorption spectroscopy in the region of valence vibrations of OH$^-$ groups, the photoluminescence in the optical spectral range, and the photoinduced light scattering have been used to study the peculiarities of the defect structure and their influence on the properties of LiNbO$_3$:Zn crystals doped in a wide concentration range including two concentration thresholds (at $\sim$ 3.0 mol.% ZnO and $\sim$ 6.8 mol.% ZnO in melt). In the LiNbO$_3$:Zn (0.004–2.01 mol.% ZnO) crystals, the concentration of hydroxyl groups increases and the luminescence intensity from the luminescence centers related to Nb$_{\mathrm{Li}}$ defects decreases as the zinc concentration increases. The latter is likely to be related to the formation of shallow energy levels near the conduction band bottom as niobium atoms are displaced by zinc atoms from the lithium positions of the ideal structure and, correspondingly, the decrease in the Nb$_{\mathrm{Li}}$ defect concentration. In highly doped LiNbO$_3$:Zn (4.46–6.5 mol.% ZnO) crystals and the LiNbO$_{\mathrm{3stoich}}$(6.0 wt.% K$_2$O) crystal, the concentration of OH$^-$ groups is markedly lower, the energy gap width increases by 0.3–0.4 eV, the luminescence intensity in the green spectral range increases due to the formation of new recombination channels as compared to the weakly doped crystals. In addition, in such crystals, the proton electrical conductivity increases due to the increase in the concentration of interstitial hydrogen H$^+$ and, as a result, the formation of set of shallow acceptor level near the valence band top.